1. Field of the Invention
The present invention relates to an exhaust gas purifying device for an engine.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 5-113116 discloses an exhaust gas purifying device for an engine in which a lean engine operation is performed, i.e., an air-fuel ratio is made lean with respect to a stoichiometric air-fuel ratio. The device is provided with a NO.sub.X catalyst arranged in an exhaust passage of the engine capable of selectively reducing NO.sub.X in the inflowing exhaust gas in the oxidizing atmosphere including a reducing agent, and a reducing agent feeding device for secondarily feeding the reducing agent to the exhaust passage upstream of the NO.sub.X catalyst. The exhaust gas of the engine performing the lean operation provides the oxidizing atmosphere including the reducing agent, such as unburned hydrocarbon (HC) and carbon monoxide (CO). Thus, if this exhaust gas is introduced to the NO.sub.X catalyst, a part of NO.sub.X in the exhaust gas is reduced and purified by the unburned HC and CO. However, the amount of the unburned HC and CO in the exhaust gas discharged from the engine is much smaller than that of NO.sub.X, and the reducing agent will be insufficient to purify NO.sub.X. Therefore, the exhaust gas purifying device mentioned above is provided with the reducing agent supplying device for secondarily feeding the reducing agent to the exhaust passage upstream of the NO.sub.X catalyst, to thereby avoid a shortage of the reducing agent with respect to NO.sub.X. Note that the NO.sub.X catalyst comprises a transition metal or a precious metal carried on a carrier, such as zeolite, by an ion changing process.
On the other hand, it is well known that an exhaust gas recirculation (EGR) system in which a part of the exhaust gas is recirculated to an intake passage of the engine as an EGR gas, and the feeding of the EGR gas to the engine, reduces the amount of NO.sub.X discharged from the engine. In this case, an EGR ratio, which is a ratio of the amount of the EGR gas to the sum of the amounts of the EGR gas and the fresh air fed to the engine, can be controlled in accordance with the engine operating condition.
However, the amounts of NO.sub.X, unburned HC, and CO will vary in accordance with the variation of the EGR ratio. Accordingly, if the EGR system is applied to the exhaust gas purifying device mentioned above and the reducing agent is simply fed, the reducing agent may be in excess or insufficient with respect to the NO.sub.X to be purified.